1. Field of the Invention
The present invention relates to a method of forming a dielectric film. More particularly, the present invention relates to a method of forming a dielectric film by using an atomic layer deposition (ALD) method and a method of manufacturing a capacitor having the dielectric film.
2. Description of the Related Art
With the advancement of a miniaturization technique, an integration density of DRAM has been increased year by year, and the area of one capacitor has been decreased. On the other hand, the capacitance of the capacitor needs to be kept, for the sake of a normal operation of a semiconductor memory device. Thus, with the advancement of a generation of DRAM, the structure having a high aspect ratio such as a cylindrical structure has been mainly used for the capacitor structure. In this case, a dielectric film (capacitive insulating film) of the capacitor needs to be formed on a lower electrode under the structure having the high aspect ratio. For this reason, as the method of forming the dielectric film, an ALD (Atomic Layer Deposition) method is used which is excellent in coverage to a conventional CVD (Chemical Vapor Deposition) method.
According to the ALD method, gaseous reactants are alternately supplied into a reaction chamber, and a film is sequentially formed in an atomic layer level. FIGS. 1A to 1E are diagrams showing a method of forming a dielectric film by using the ALD method. For example, an amorphous aluminum oxide film is formed as the dielectric film. TMA (Tri-Methyl Aluminum) is used as a first material gas including aluminum as the material of the aluminum oxide film, for example. Also, O3 (ozone) gas is used as a second material gas (oxidization gas) necessary for oxidization reaction.
At first, as shown in FIG. 1A, a TMA gas 2 as the material gas is introduced into the reaction chamber. Thus, the TMA gas 2 is chemically adsorbed on the surface of a lower layer 1. The TMA gas 2 that is not adsorbed is exhausted from the reaction chamber through a purging operation by using inert gas such as argon (Ar) gas or an evacuating operation. As a result, as shown in FIG. 1B, a TMA film 3 is formed on the surface of the lower layer 1. Next, as shown in FIG. 1C, an ozone gas 4 as an oxidization gas from which oxygen radicals can be generated is introduced into the chamber. Since the ozone gas 4 chemically reacts with the TMA film 3 to form an aluminum oxide. Substances such as hydrogen and carbon that are included in the TMA 2 and are not absorbed on the lower layer 1, are exhausted from the reaction chamber. As a result, as shown in FIG. 1D, an aluminum oxide film 5 is formed on the surface of the lower layer 1. This aluminum oxide film 5 is an atomic layer having a film thickness of a substantial atomic size of a desirable substance.
Hereafter, since the TMA gas 2 and the ozone gas 4 are alternately supplied and the similar steps re repeated, the aluminum oxide films 5 are sequentially laminated. As a result, as shown in FIG. 1E, a dielectric film 6 having a desirable film thickness is formed on the lower layer 1. In this way, according to the ALD method, a thin film is formed by increasing the atomic layer one by one. As a result, even in case of the high aspect structure, it is possible to grow the thin film that is fine and excellent in quality.
However, in case of the ALD method, the quality of the film formed at an initial stage is greatly influenced by the state of the surface of the lower layer 1. For example, if an organic material is deposited on the surface of the lower layer 1 or if a natural oxide film is locally grown, the surface of the lower layer 1 becomes ununiform. In that case, the film low in density and including many impurities so that the quality is poor is formed at the initial stage of the dielectric film formation. This leads to deterioration in characteristics of a semiconductor device using the formed dielectric film. Also, it is known that in the boundary of the formed dielectric film, any defect of a crystal structure caused due to dangling bonds is generated, which leads to the deterioration in characteristics of the semiconductor device.
For example, in case of the DRAM, when the dielectric film (capacitive insulating film) of the capacitor has a defect, or if its film quality is poor, leakage current from the capacitor increases. The increase in leakage current results in the deterioration in data holding characteristic of the DRAM, and a frequently refreshing operation leads to the increase in power consumption. Thus, in order to prevent the deterioration in DRAM characteristics, the dangling bond, the organic material deposited on the lower electrode surface, and the natural oxide film need to be sufficiently removed. In particular, in order to improve the characteristics of the high integration DRAM, a technique is demanded that can provide the capacitive insulating film less in defect and excellent in quality by the ALD method.
In conjunction with the above description, a method of manufacturing a capacitor that has a double dielectric film structure is described in Japanese Laid Open Patent Application (JP-P2003-318284A). In this conventional manufacturing method, a lower electrode is formed on a semiconductor substrate, a double dielectric film composed of an Al2O3 film and a Ta2O5 film is formed on the lower electrode, and an upper electrode is formed on the double dielectric film. Prior to the evaporation of the Al2O3 film, a low temperature heating process is temporally carried out in NO2 or O2 ambience by using in-situ plasma for selective oxidization.
Also, a method of forming a thin film is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 7-321046). In this conventional method, an ultraviolet lamp for cleaning a wafer and an infrared lamp for heating are provided in a film forming chamber in an LPCVD apparatus. After a sample substrate is fed into the film forming chamber, firstly, an O3 gas is introduced while ultraviolet rays are irradiated by the ultraviolet lamp. Thus, the organic material on the wafer surface is removed. In succession, a mixture gas of H2 and F2 is introduced while the ultraviolet rays are irradiated. Therefore, a natural silicon oxide film is removed. After that, the irradiation of the ultraviolet rays is stopped, and the output of the infrared lamp is increased. After a substrate temperature is increased, the thin film is formed by an LPCVD method.
According to the techniques disclosed in the above conventional examples, a special mechanism needs to be provided in the film forming apparatus. Thus, there is no general purpose.
Also, a method of forming a high dielectric film that is used as a gate insulating film of a MOS transistor is disclosed in Japanese Laid Open Patent Application (JP-P2003-188171A). In this conventional method, a silicon substrate is oxidized through an oxidative gas containing ozone, and a first reactive material is adsorbed on the oxidization surface of the substrate. Then, a second reactive material is introduced to react with the residue of the first reactive material on the oxidization surface. At the first step, the ozone gas (O3) of a high concentration that is diluted with oxygen gas (O2) is introduced into a reacting chamber. The concentration of the ozone gas diluted with the oxygen gas is, for example, 70%. In this first step, a relatively thick oxide film of the film thickness of 0.1 to 1 nm is formed on the substrate surface. However, if such a thick capacitive oxide film is formed in manufacturing a capacitor, the capacitance of the capacitor is greatly reduced.